Metagenomic studies have become increasingly popular. They allow for the estimation of biodiversity in complex populations. This diversity presents an enormous but largely unexpected genetic and biological pool and can be exploited for the recovery of novel genes, entire metabolic pathways and their products. Generally metagenomic study is a genomic analysis of organisms by direct extraction and cloning of DNA from their natural environment. The most common problems of modern metagenomics are as follows: majority of the microorganisms present in the environment cannot be cultivated by standard techniques, DNA extraction methods are not very effective, isolated DNA is contaminated with various compounds, a choice for a screening method is not obvious.

A microRNA-size (20-nt long) molecule has been identified in Escherichia coli after induction of Shiga toxin-converting bacteriophage Φ24_B. This small RNA, named 24B_1, is encoded in the lom-vb_24B_43 region of the phage genome and apparently it is produced by cleavage of a larger transcript. A phage devoid of 24B_1 revealed decreased efficiency of lysogenization, quicker prophage induction after provoking the SOS response, higher efficiency of progeny phage production during the lytic cycle and less efficient adsorption on the host cells. Expression of most of phage genes was drastically increased after infection of E. coli by the Φ24_BΔ24B_1 phage. Since 24B_1 may impair expression of the d_ant gene, coding for an anti-repressor, these results may explain the mechanism of regulations of the physiological processes by this small RNA due to impaired activity of the cI repressor and changed expression of vast majority of phage genes. To our knowledge, this is the first example of functional microRNA-size molecule in bacterial cells.

Shiga toxin-producing Escherichia coli (STEC) strains belong to the group of pathogens that cause bloody diarrhea and hemorrhagic colitis with often severe complications. The main problem with human pathogenic E. coli strains, including STEC, is a wide spectrum of phenotypes and clinical manifestations. It is related to a variety of exchangeable genetic elements, like plasmids, bacteriophages, transposons and pathogenicity islands, that take part in horizontal gene transfer which influences creation of new dangerous bacterial strains. A good example of this phenomenon is a novel Shiga toxin-producing E. coli O104:H4 serotype that was associated with a widespread and severe foodborne disease outbreak in Germany in 2011. The O104:H4 strain was created by a number of horizontal gene transfer events between two distinct pathogens, resulting in the emergence of the new, atypical strain. That outbreak proved that also rare and unusual serotypes of STEC may be a significant risk factor and that the procedures recommended for STEC detection were not suitable to deal with this kind of pathogens. With respect to new combinations of chromosomal and extrachromosomal elements in susceptible bacterial hosts, epidemics and frequent human infections caused by STEC strains, we suggest that more attention should be paid to the development and improvement of diagnostic methods. It is difficult to determine STEC bacteria by general microbiological, biochemical and immunological assays, because strains can vary dramatically in their phenotypic and serotypic properties. It is postulated that standardized genetic tests, based on detection of features most frequently presented by STEC, particularly those located on easily exchangeable elements (such as Shiga toxin-encoding phages), can be more adequate for STEC detection.